The present invention relates to a process for preparing apatite and more particularly to a process for preparing a calcium-phosphorus apatite.
Apatite is a general term for a wide range of compounds represented by the general formula M.sup.2+.sub.10 (ZO.sub.4.sup.3-).sub.6 Y.sub.2.sup.- wherein M is a metal atom such as, for example, Ca, Pb, Ba, Sr, Mg, Ni, Na, K, Fe or Al, ZO.sub.4 is an acid radical such as, for example, PO.sub.4, AsO.sub.4, VO.sub.4, SO.sub.4, SiO.sub.4 or CO.sub.3, and Y is an anionic atom (or group) such as, for example, F, OH, Cl, Br, .dbd.O or .dbd.CO.sub.3.
Compounds to be prepared by the process of the present invention are those of the above general formula wherein M is substantially Ca and ZO.sub.4 is substantially PO.sub.4. Therefore, the term "calcium-phosphorus apatite" is used in the present invention, and this term will be hereinafter referred to simply as "Ap". In the present invention, moreover, Ap wherein Y is substantially OH is referred to as "hydroxyapatite", which will be hereinafter referred to simply as "HAp", and Ap wherein ZO.sub.4 and/or Y are partially substituted by .dbd.CO.sub.3 is referred to as "carbonate-containing hydroxyapatite", which will be hereinafter referred to simply as "COAp".
Ap has recently been attracting special attention as a raw material of bioceramics, and its application to artificial bones and teeth has been studied. Further, as a laser material, a catalyst for the dehydration of alcohols, a fluorescent material, an electronic material, an inorganic ion exchanger, and also as a packing material for chromatography for the purpose of separation of polymeric substances such as proteins, nucleic acids, enzymes and viruses by utilization of a high biocompatibility of Ap, Ap is now considered to be promising.
More particularly, it is a well-known fact as described in many literatures that natural teeth and natural bones are composite materials of about 70% HAp and about 30% organic substances such as collagen. Of great interest is the fact that the HAp as a constituent of natural teeth and natural bones contains several percent of carbonate radicals. And HAp prepared so as to contain several percent of carbonate radicals, namely, COAp, is attracting considerable attention as a raw material of bioceramics. Many advantages are expected by using COAp similar in chemical structure to natural teeth or natural bones as an implant material, for example, an enhanced biocompatibility, a faster acclimation to the natural teeth or natural bones, and a decreased rejection.
As to the manufacturing process for Ap having such advantages, various processes have heretofore been disclosed in literatures, patents, etc., including the following main processes:
(1) Hydrothermal process involving reacting anhydrous calcium hydrogenphosphate with phosphoric acid in an autoclave at 100.degree.-500.degree. C., 1-500 atm. for about 48 hours. PA1 (2) Dry process involving reacting tri-calcium phosphate with calcium oxide at a high temperature of 900.degree.-1300.degree. C. under a stream of steam for about 3 hours. PA1 (3) Wet process involving reacting a water-soluble phosphate with a water-soluble calcium salt in an aqueous solution at 37.degree. C. and at a pH value of 7-8 for at least 20 days to obtain Ap having an approximately stoichiometric ratio.
However, in both the above processes (1) and (2), since the reactions are carried out at high temperatures and high pressures, the synthesis apparatus are inevitably expensive; besides, the operation of the apparatus is complicated and the amount of energy consumed is very large. In the above process (3), even a slight difference in the reaction conditions greatly affects the composition of the resulting product, and such a long period of 20 days, which is inconceivable in an industrial-scale production, is required for obtaining Ap of a stoichiometric composition, thus resulting in that the handling of the materials and the operation of the apparatus become complicated, and the reproducibility of results is not good.
Due to these drawbacks, with the conventional processes it has been very difficult to prepare Ap of good quality quantitatively in practical amounts, and because of this difficulty in the production of Ap, the resulting product becomes very expensive and the use thereof is limited to a great extent although the superior properties of Ap are known.